When iPhone is not connected
Open iTunes and mouse over a device backup:
Open Preferences in iTunes. (In Mac OS X, choose iTunes > Preferences. In Windows, choose Edit > Preferences.)
Click the Devices tab.
Position the mouse over a backup to display the phone number, serial number, and IMEI or MEID of the backed-up iPhone.
You can also hold down the Control key and choose Help > About iTunes (Windows) or iTunes > About iTunes (Mac).
Release the Control key.
As the iTunes and QuickTime version information scrolls, you will see the last connected iPhone's serial number and IMEI or MEID:
About iTunes window